> However for physics reasons the atmosphere does not hold much water vapor.
The atmosphere is currently ~422 ppm of CO2, and an average of ~5,000 ppm of H2O, with humid tropical areas having up to ~50,000 ppm of H2O.
Water vapour makes up ~half of the greenhouse effect. It's not a 'good' greenhouse gas, it's just that there's a lot of it in the parts of the world that get a lot of sun - the tropics.
I am definitely not an expert on the subject. But doing some basic web searches and I note how much more of the infrared spectra is absorbed by water vs the relatively narrow band that co2 absorbs. As a infrared emission blocking gas water vapor is several times more effective than co2. this is fine, we don't want earth to be a freezing ice cube. but add the excessive amounts of co2 and you start to get an alarming condition.
In fact if I understand correctly this is the mechanism for a runaway greenhouse, that is, something like what happened to venus. Warmer temps allow for more water vapor in the atmosphere, due to it being such a good greenhouse gas, temps then build out of control as more and more water is vaporized.
https://en.wikipedia.org/wiki/Greenhouse_gas#Role_of_water_v...